Mortgage-Backed Securities - Unrealized Loss Positions (Detail) - Residential Mortgage-Backed Securities $ in Thousands	Dec. 31, 2016 USD ($) security	Dec. 31, 2015 USD ($) security
Investment Holdings [Line Items]
Estimated fair value of RMBS in continuous loss position for less than 12 consecutive months	$ 3,815,885	$ 5,477,895
Unrealized losses on RMBS in continuous loss position for less than 12 consecutive months	$ (110,548)	$ (92,223)
Number of Securities of RMBS in continuous loss position for less than 12 months | security	846	406
Estimated fair value of RMBS in continuous loss position for 12 or more consecutive months	$ 252,257	$ 403,312
Unrealized losses on RMBS in continuous loss position for 12 or more consecutive months	$ (59,127)	$ (50,267)
Number of Securities on RMBS in continuous loss position for 12 or more consecutive months | security	156	64
Estimated fair value of RMBS in continuous loss position	$ 4,068,142	$ 5,881,207
Unrealized losses on RMBS in continuous loss position	$ (169,675)	$ (142,490)
Number of Securities in continuous loss position | security	1,002	470
Non-Agency RMBS - Senior
Investment Holdings [Line Items]
Estimated fair value of RMBS in continuous loss position for less than 12 consecutive months	$ 12,384	$ 294,520
Unrealized losses on RMBS in continuous loss position for less than 12 consecutive months	$ (250)	$ (8,779)
Number of Securities of RMBS in continuous loss position for less than 12 months | security	3	20
Estimated fair value of RMBS in continuous loss position for 12 or more consecutive months	$ 0	$ 0
Unrealized losses on RMBS in continuous loss position for 12 or more consecutive months	$ 0	$ 0
Number of Securities on RMBS in continuous loss position for 12 or more consecutive months | security	0	0
Estimated fair value of RMBS in continuous loss position	$ 12,384	$ 294,520
Unrealized losses on RMBS in continuous loss position	$ (250)	$ (8,779)
Number of Securities in continuous loss position | security	3	20
Non-Agency RMBS - Senior interest-only
Investment Holdings [Line Items]
Estimated fair value of RMBS in continuous loss position for less than 12 consecutive months	$ 96,399	$ 81,919
Unrealized losses on RMBS in continuous loss position for less than 12 consecutive months	$ (13,600)	$ (18,715)
Number of Securities of RMBS in continuous loss position for less than 12 months | security	62	83
Estimated fair value of RMBS in continuous loss position for 12 or more consecutive months	$ 78,516	$ 64,058
Unrealized losses on RMBS in continuous loss position for 12 or more consecutive months	$ (43,931)	$ (33,742)
Number of Securities on RMBS in continuous loss position for 12 or more consecutive months | security	86	47
Estimated fair value of RMBS in continuous loss position	$ 174,915	$ 145,977
Unrealized losses on RMBS in continuous loss position	$ (57,531)	$ (52,457)
Number of Securities in continuous loss position | security	148	130
Non-Agency RMBS - Subordinated
Investment Holdings [Line Items]
Estimated fair value of RMBS in continuous loss position for less than 12 consecutive months	$ 56,015	$ 138,257
Unrealized losses on RMBS in continuous loss position for less than 12 consecutive months	$ (412)	$ (7,577)
Number of Securities of RMBS in continuous loss position for less than 12 months | security	7	22
Estimated fair value of RMBS in continuous loss position for 12 or more consecutive months	$ 2,826	$ 0
Unrealized losses on RMBS in continuous loss position for 12 or more consecutive months	$ (824)	$ 0
Number of Securities on RMBS in continuous loss position for 12 or more consecutive months | security	4	0
Estimated fair value of RMBS in continuous loss position	$ 58,841	$ 138,257
Unrealized losses on RMBS in continuous loss position	$ (1,236)	$ (7,577)
Number of Securities in continuous loss position | security	11	22
Non-Agency RMBS - Subordinated interest-only
Investment Holdings [Line Items]
Estimated fair value of RMBS in continuous loss position for less than 12 consecutive months	$ 748	$ 6,455
Unrealized losses on RMBS in continuous loss position for less than 12 consecutive months	$ (230)	$ (1,039)
Number of Securities of RMBS in continuous loss position for less than 12 months | security	2	1
Estimated fair value of RMBS in continuous loss position for 12 or more consecutive months	$ 11,276	$ 3,635
Unrealized losses on RMBS in continuous loss position for 12 or more consecutive months	$ (1,624)	$ (2,996)
Number of Securities on RMBS in continuous loss position for 12 or more consecutive months | security	3	2
Estimated fair value of RMBS in continuous loss position	$ 12,024	$ 10,090
Unrealized losses on RMBS in continuous loss position	$ (1,854)	$ (4,035)
Number of Securities in continuous loss position | security	5	3
Agency MBS - Residential
Investment Holdings [Line Items]
Estimated fair value of RMBS in continuous loss position for less than 12 consecutive months	$ 2,338,910	$ 4,468,717
Unrealized losses on RMBS in continuous loss position for less than 12 consecutive months	$ (48,084)	$ (44,687)
Number of Securities of RMBS in continuous loss position for less than 12 months | security	106	116
Estimated fair value of RMBS in continuous loss position for 12 or more consecutive months	$ 54,943	$ 290,926
Unrealized losses on RMBS in continuous loss position for 12 or more consecutive months	$ (1,615)	$ (7,314)
Number of Securities on RMBS in continuous loss position for 12 or more consecutive months | security	1	4
Estimated fair value of RMBS in continuous loss position	$ 2,393,853	$ 4,759,643
Unrealized losses on RMBS in continuous loss position	$ (49,699)	$ (52,001)
Number of Securities in continuous loss position | security	107	120
Agency MBS - Commercial
Investment Holdings [Line Items]
Estimated fair value of RMBS in continuous loss position for less than 12 consecutive months	$ 1,247,923	$ 393,058
Unrealized losses on RMBS in continuous loss position for less than 12 consecutive months	$ (45,802)	$ (7,969)
Number of Securities of RMBS in continuous loss position for less than 12 months | security	646	140
Estimated fair value of RMBS in continuous loss position for 12 or more consecutive months	$ 51,733	$ 4,986
Unrealized losses on RMBS in continuous loss position for 12 or more consecutive months	$ (4,035)	$ (32)
Number of Securities on RMBS in continuous loss position for 12 or more consecutive months | security	46	4
Estimated fair value of RMBS in continuous loss position	$ 1,299,656	$ 398,044
Unrealized losses on RMBS in continuous loss position	$ (49,837)	$ (8,001)
Number of Securities in continuous loss position | security	692	144
Agency MBS - Interest-only
Investment Holdings [Line Items]
Estimated fair value of RMBS in continuous loss position for less than 12 consecutive months	$ 63,506	$ 94,969
Unrealized losses on RMBS in continuous loss position for less than 12 consecutive months	$ (2,170)	$ (3,457)
Number of Securities of RMBS in continuous loss position for less than 12 months | security	20	24
Estimated fair value of RMBS in continuous loss position for 12 or more consecutive months	$ 52,963	$ 39,707
Unrealized losses on RMBS in continuous loss position for 12 or more consecutive months	$ (7,098)	$ (6,183)
Number of Securities on RMBS in continuous loss position for 12 or more consecutive months | security	16	7
Estimated fair value of RMBS in continuous loss position	$ 116,469	$ 134,676
Unrealized losses on RMBS in continuous loss position	$ (9,268)	$ (9,640)
Number of Securities in continuous loss position | security	36	31